Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
As new applications and services are being offered/consumed over wireless devices, there is an increasing need for high data rate in order to provide better customer satisfaction. At present, there are several communication standards such as orthogonal frequency division multiplexing (OFDM), single carrier-frequency division multiple access (SC-FDMA), and discrete Fourier transform pre-coded orthogonal frequency division multiple access (DFT-pre-coded-OFDMA), among others that aim to achieve high data rates. In wireless communication standards such as OFDM and OFDMA, a signal is “spread out” and distributed among subcarriers, which send portions of the signal in parallel. High data rate is achieved by sending a signal in parallel over all possible channels available between the sender device, interchangeably referred to as transmitter hereinafter, and the receiver device, interchangeably referred to as receiver hereinafter. To minimize interference and loss of data during transmission, subcarrier frequencies are chosen in such as a way that the modulated data streams are orthogonal to each other, and cross-talk between the sub-channels is eliminated so that inter-carrier guard bands are not required. At receiver side, the receiver receives and reassembles the data that is sent in parallel over different channels and/or sub-channels by the transmitter.
These existing standards, also interchangeably referred to as protocols hereinafter, that are in use today, suffer from a high peak-to-average power ratio (PAPR), where PAPR of the symbols being transmitted using any of these standards is very high. PAPR, which is ratio of Peak Power with respect to Average Power of all the symbols that will be or are transmitted by the transmitter, is a performance parameter for measuring performance and/or efficiency of any transmitter/transceiver. PAPR is therefore the ratio of Peak Power with respect to Average Power of all symbols that will be or are transmitted, wherein PAPR is a metric used to measure transmission efficiency of the RF power amplifier present within the Radio Transmitter. PAPR is the peak amplitude squared (giving the peak power) divided by the RMS value squared (giving the average power), also calculated as square of crest factor. PAPR is a metric used to measure transmission efficiency of the RF power amplifier that is present within a radio transmitter, wherein ideal PAPR value of transmitted symbols by any transmitter should be one, and a high PAPR value dictates use of a linear transmit chain to avoid signal distortion that results in degraded error performance and spectral re-growth beyond intended signal bandwidth. In particular, power amplifier (PA) characteristics of the transmitter exhibit a saturation of output power with increased input power and hence present a nonlinear behavior. To ensure linearity with high PAPR, PA of the transmitter is operated away from saturation, i.e. with a power back-off. Operating with a power back-off results in decreased transmission range of the transmitter and reduced power efficiency of the transmitter. Low PAPR value is also required for increased transmission range of the transmitter and to reduce the power consumption by the transmitter. By lowering the PAPR value, infrastructure cost for setting-up dense transmitter/transceiver network can also be controlled.
In existing Single Carrier Transmission (SCT) systems, transmitted signal or waveform is band limited but consists of infinite frequencies, where these infinite frequencies present within the band can be called as ‘Sub-carriers’ to differentiate between the high-frequency ‘Carrier’ that is used for modulation by the signal. These infinite sub-carriers lead to significant interference between different or adjacent sub-carriers or frequency channels or frequency components and leads to errors, e.g. Inter-Symbol Interference (ISI). On the other hand, in Multi Carrier Transmission (MCT) systems, the transmitted signal or waveform is band-limited and there are a finite number of sub-carriers or frequency channels or frequency components. However, the Peak-to-average Power Ratio (PAPR) is very high. For instance, in Orthogonal Frequency Division Multiplexing (OFDM) system, which is an example of MCT systems, the transmitted signal or waveform is band-limited and there are a finite number of sub-carriers or frequency channels or frequency components that are all orthogonal to each other. Orthogonality reduces cross-talk between different frequency components, however, the Peak-to-average Power Ratio (PAPR) is still very high.
As the cost of setting-up and maintenance of access points/transmitters is increasing, it is better to utilize transmitters to their maximum/optimal potential, and use existing transmitters in the most efficient manner possible, and hence one of the primary objectives for any wireless communication scheme is to lower the PAPR of the transmitted symbols.
In prior art solutions, in order to achieve better PAPR, different techniques have been proposed including coding techniques, constellation reshaping, tone-reservation, and selective mapping, to name a few. For instance, amplitude clipping can be directly applied to reduce the PAPR. However, this clipping results in in-band and out-of-band distortions, which results in Symbol-Error-Rate (SER) degradation and out-of-band radiation respectively. To counter the effect of out-of-band distortions, a filter can be applied to the clipped signal. However, this might also regenerate new peaks. Hence, amplitude clipping reduces the PAPR at the expense of quantifiable distortion.
In another approach to reduce the PAPR, symbols are mapped into code words, and extra bit(s) are padded/added to those code words, and only code words that do not result in high PAPR are chosen for transmission. This technique requires lookup tables and exhaustive search for the best code word. Another approach known as selected mapping reduces PAPR by generating different sets of data blocks and transmitting one with the lowest PAPR. This is done by multiplying the initial data set with different phase sequences, and the optimal phase sequence is sent separately to the receiver as side information. A similar approach known as the Interleaving has also been used in past that uses interleaver instead of a set of phase sequences to produce different sequences of the same data and transmits the one with the minimum PAPR.
Some methods use extra tones to add a peak-reducing signal to the original multicarrier signal to reduce the overall PAPR. This signal can be stripped off at the receiver using the information available at the reserved tones. However, none of the foregoing techniques have proven entirely satisfactory.
Another similar technique is proposed in SC-FDMA standard that performs a Fourier transform on the signal before mapping and sending the signal over the subcarriers to send it through a serial transmission (rather than in parallel transmission as used by ODFM). On reception of the transmission, an inverse Fourier transform is performed. Though, the SC-FDMA scheme offers a lower PAPR than the OFDM and OFDMA, effectiveness and/or efficiency of SC-FDMA scheme is limited by the choice of mapping scheme employed. Performance of SC-FDMA also suffers due to serial transmission.
Though several techniques/schemes as cited above have been proposed in the past, none of the foregoing techniques have proven entirely satisfactory. Therefore, there still exists a need for communication systems and methods for achieving low PAPR values for symbols transmitted by any transmitter.